1. Field of the Invention
The present invention relates to the field of semiconductor integrated circuit (IC) manufacturing, and more specifically, to an apparatus for automatically tilting a sample to a desired angle for processing, measurement, or inspection.
2. Discussion of Related Art
During semiconductor IC manufacturing, failure analysis (FA) of a product is often performed at various stages of processing to locate failures and identify causes. The FA may include physical or chemical techniques. A common type of physical FA is to cross-section a Silicon wafer containing the product for analysis in a transmission electron microscope (TEM). A TEM can resolve structural details in the devices that are on the order of 2 Angstroms.
TEM samples may be prepared with various orientations to reveal different parts of the device structure. Examples are shown in FIG. 1(a) and FIG. 1(b). A TEM sample 130 has a length 110 of about 3 millimeters, a height 112 of about 1 millimeter, and a depth 114 of about 20 micrometers. The TEM sample 130 has a device layer 103 formed on a substrate 105. A platinum cap 107 is deposited over a region of interest 116. Ion milling is used to remove material from one side 133 of the Platinum cap 107. Then ion milling is used to remove material from the other side 136 of the Platinum cap 107. As a result, an area of interest 116 in the sample 130 is reduced to a thickness 118 of about 1000 Angstroms so electrons can penetrate to form an image. The platinum cap 107 is subsequently removed.
In a dual-beam focused ion beam (FIB) tool from FEI corporation, as shown in FIG. 2(a), an ion beam 220 is used to sputter away material from the sample 230 while, at the same time, an electron beam 225 is used to monitor the progress. The electron beam 225 points downwards vertically while the ion beam 220 points downwards with an angle 265 from the vertical. In the XL820 model of the tool, the angle 265 is fixed at 52 degrees.
FIG. 2(a) shows a sample 230 that is clamped in a vice 240 attached to a stage 260. The sample 230 should be parallel to the ion beam 220 in order to be thinned down properly from the top down. In order to align the sample 230 with the ion beam 220, a stage 260 on which the sample 230 rests must be tilted at an angle 275. If the sample 230 is mounted perpendicularly relative to the stage 260, as is desired, then the tilt angle 275 will be equal to a sample rotation angle 270.
The sample 230 may be inadvertently misaligned in a vice 240 by an angle 280 from the vertical, as shown in FIG. 2(b). In such a case, the sample 230 would not be mounted perpendicularly relative to the stage 260.
One problematic consequence of sample misalignment is that the ion beam 220 may not be able to mill one side 233 of the sample 230 straight down since the tilt angle 275 of the XL820 stage 260 cannot go beyond 52 degrees. Another problem is that the electron beam 225 may not be able to monitor the progress of the ion milling if the side 233 being ion milled is not directly in the line of sight of the electron beam 225. Milling xe2x80x9cblindlyxe2x80x9d may result in poor control of depth and symmetry.
A similar problem may arise with misalignment of a sample 230 that is being converted from analysis with a scanning electron microscope (SEM) to analysis with a TEM. In a TEM sample, a deposited platinum cap 107 protects the top surface directly over a region of interest 116, as shown in FIG. 1(a) and FIG. 1(b). However, a sample previously analyzed with a SEM will have platinum covering the face (not shown), as well as the top surface, of the region of interest 116. As a result, the additional platinum protecting the face of the region of interest 116 cannot be milled of symmetrically since the stage 260 of the XL820 is unable to tilt to an angle 275 that exceeds 52 degrees.
A possible solution for handling a misaligned sample 230 is to modify the stage 260 to tilt at an angle 275 that is beyond the mill angle 265 of 52 degrees. However, such a modification would be costly and difficult to implement.
Thus, what is needed is an apparatus for automatically tilting a sample to a desired angle for processing, measurement, or inspection.